In the processing of molten metals, for example aluminum and zinc, it is often necessary to pump molten metal from one vessel to another. When the molten metal needs to be removed from a vessel by elevating it over a containment wall, a so-called transfer pump is often used. Most typical of this situation is where the transfer pump is placed in the charge well of a molten metal furnace (see FIG. 1) to remove molten metal from the furnace. Of course, the present invention is not limited to any particular application for a transfer pump.
A problem unique to the design of transfer pumps is the assembly of the riser and its mating to the discharge piping. Particularly, as those skilled in the art understand, the high temperatures to which the riser assembly is exposed and the inherent temperature cycling experienced, place unusual stress on the assembly. More particularly, the riser tube must be constructed of a refractory material to allow for it's submergence in the molten metal bath, while the discharge piping is preferably constructed of a ceramic lined metallic material to provide high strength. Unfortunately, the coupling of these divergent materials can be problematic because of different rates of thermal expansion and comparative strengths.
In the prior art, as demonstrated by the schematic of FIG. 2, a riser is usually cemented at a first end to a pump base assembly and to a riser socket at a second opposed end. The riser socket is then bolted to a motor mount, and the second end of the riser extends slightly above the riser socket to provide a mating surface with the discharge piping assembly.
As will be recognized, this design places a great deal of angular stress on the riser due to the weight of the ceramic lined pipe, flanges and the contained molten metal positioned in a generally tangential direction to the riser. Additionally, the riser must withstand a gasket seating stress which is often exaggerated by uneven or over tightening of the fasteners.
In addition, the riser also shares the stresses experienced by the posts which suspend the base from the motor mount. Compounding this problem is the fact that the riser experiences rapid thermal changes caused by intermittent transferring of metal while the posts see only a steady thermal state.
In summary, since the refractory material used to form the riser has a relatively low tensile strength and is subjected to the aforementioned combination of stresses, the riser in the vicinity of the coupling assembly is typically one of the most frequent points of failure in transfer pumps.